Method and apparatus to reduce interference in a communication device

Abstract

An apparatus and method to reduce interference in a communication device particularly effective for second-order interference in direct conversion receiver from a co-located transmitter. This is accomplished by characterizing a transfer function of a transmission path from the transmitter to the receiver and applying this transfer function to the baseband signal from the transmitter to provide an estimation of interference to be expected in the received signal. The estimated interference is subtracted from the receiver baseband signal to reduce interference. Adaptive filtering can also be applied to further minimize interference dynamically.

Description

FIELD OF THE INVENTION[0001]The present invention relates to operation of a radio communication device. More particularly the invention relates to a method and apparatus to reduce interference in a radio communication device.BACKGROUND OF THE INVENTION[0002]In a two-way radio, the frequency bands for receive and transmit operations may overlap for the radio. In this case, when the radio transmitter is operative, signals from the transmitter may be coupled to the radio receiver, causing interference. Although interference can happen when the transmitter and receiver are operating on overlapping carrier frequencies, interference can also occur on interfering intermediate frequencies or baseband frequencies. Direct conversion radios are particularly susceptible to the latter problem. Differences between operational modes may result from the receiver architecture (e.g., superheterodyne), system design (e.g., separate frequency blocks for transmit and receive signals) or transmitter arch...

AI Technical Summary

Problems solved by technology

In this case, when the radio transmitter is operative, signals from the transmitter may be coupled to the radio receiver, causing interference.

Although interference can happen when the transmitter and receiver are operating on overlapping carrier frequencies, interference can also occur on interfering intermediate frequencies or baseband frequencies.

Direct conversion radios are particularly susceptible to the latter problem.

In those cases where the downlink frequency is close to the uplink transmission frequency, the communication device can actually interfere with itself.

In other words, the transmit power of the device is picked up by, and interferes with, the receiver of the device.

In addition, radio self-interference can occur in a Global Positioning System (GPS) wherein the time to correlate a received GPS system signal is long.

In this case, the prior art solution is to mute the device transmitter during receiver correlation, which is not acceptable as it limits the use of the device.

Otherwise, intermodulating frequency by-products appear in the form of unwanted spurious signals in an operating passband or receive band of the radio.

In particular, where a direct conversion or VLIF receiver operates simultaneously with a co-located transmitter with non-constant envelope modulation, any transmitter signals coupling with the receiver mixer will produce second-order distortion products.

In addition, if a received signal level exceeds the operating range of the baseband circuitry of the radio, the receiver performance degrades as a result of the decreasing signal to noise ratio and receiver selectivity.

This may occur when interfering signals are very strong compared to the desired on-channel signal and the baseband circuitry becomes saturated as a result of the overload.

This results in the desired on-channel signal becoming desensitized.

However, in direct conversion or VLIF receivers, filtering is of little use as the incoming signal prior to the baseband circuitry comprises the desired monitored signal as well as the interfering signal.

However, the latter may entail reduced gain prior to the mixer, which tends to decrease sensitivity.

These prior art means tend to increase cost, weight, size, or decrease performance of the receiver.

However, periodic calibration is still not suitable for the dynamic changes in interference that occurs when an antenna interacts with its surroundings in a mobile environment.

In addition, significant DC accuracy is required and DC coupling is required.

This is not compatible with direct conversion receivers which strip away the DC component.

This is not compatible with the new third generation carrier signals that are amplitude modulated, or those continuous envelope systems with high noise in the receiver IF.

Images